Mercury in fish linked to heart attacks Consumption of fish andfish oil (which contain omega three fatty acids) has long been hypo- thesised to prevent heart attacks. A significant number of Sri Lankan doctors recommend intake of fish oil capsules with a view to prevent heart attacks and fish is a part of Mediterranean diet which is believed to reduce the incidence of heart attacks. Fish oils are prescribed by some Sri Lankan doctors for the management of hypertri -glyceridaemia (high serum levels of triglycerides) which is a risk factor for heart attacks.

It has been suggested that long term exposure to low levels of mercury predisposes to atherosclerosis by promoting the production of free radicals or by inactivating several antioxidant mechanisms (Clarkson, Toxicology of Mercury 1997). Mercury may bind to selenium to form mercury selenide which is insoluble thus neutralising the beneficial effect of selenium in prevention of atherosclerosis. Mercury may neutralise the antioxidant properties of glutathione catalase and super-oxide. Mercury may induce lipid peroxidation and mercury levels are a strong predictor of oxidised low density lipoprotein in the Kuopio Ischaemic Heart Disease Study. Mercury may promote platelet aggregability and blood coagulability. It has been suggested that mercury in fish may counteract the beneficial cardiovascular effects of omega three fatty acids in fish.

A scientific report in 1995 revealed an increased risk of heart attacks among residents of Kuopio in Finland. It is noteworthy that there is a very high consumption of fish in people of Finland.

Mercury is a highly reactive heavy metal and exposure to toxic levels of mercury results in damage to the nervous system and kidneys. Fish intake is a major source of exposure to mercury mainly in the form of methyl mercury.

Mercury exisits in three forms (a) element mercury (b) inorganic mercury compounds and (c) organic mercury, primarily methyl mercury. Exposure to inorganic mercury occurs occupationally. People can also be exposed to inorganic mercury from silver mercury dental fillings.

Exposure to methyl mercury results exlusively from the consumption of fish, shellfish (prawns, crabs and lobsters) and marine animals. These foods are a major source of mercury for the general population. Large fish such as sharks and sword fish have very high concentrations of mercury (around 1 mgm per gram), tuna, trout, pike and bass have intermediate concentrations (0.1 to 0.5 mgm per gram) and most shellfish have lower concentrations.

Increased rates of heart attacks were found among mercury exposed workers (Barrigard et al, Br. J. Ind. Med, 1990, 47, 99-104). Mercury levels were quite high in Spanish heart patients and Spanish have a very high consumption of fish.

The us Food and Drug Administration (March 2001) advised pregnant women and women who may become pregnant not to eat sword  fish, king mackeral and shark.

The toe-nail mercury level was directly associated with risk of heart attacks and the adipose tissue D. H. A. levels were inversely associated with the risk of heart attacks. Therefore high mercury content of fish may diminish the cardio protective effect of fish oil. The authors in the above study do not advise people to avoid fish.

As far as I am aware, the mercury levels of Sri Lankan heart patients have not yet been studied. As there is a significant proportion of Sri Lankan heart attack patients who don't have known risk factors such as smoking, hypertension hyperlipidaemia (high serum LDL cholesterol, high serum triglycerides low HDL levels etc), diabetes, obesity and lack of exercise, it may be worthwhile for our researchers to measure mercury levels in our heart attack patients and compare with a control group without heart disease.

-  Dr. D. P. Atukorale